Waxy hydrocarbon oils have long been dewaxed to improve their pour points and to make them useful as basestock oils for lubricating oils and other specialty oils such as refrigeration oils, white oils, turbine oils, electrical insulating oils, etc.
The wax is removed from said oils by chilling the oils to induce wax crystallization. With very light oils, this chilling can be practiced simply by reducing the temperature of the oils. However, with heavier oils it is necessary to utilize various solvents both as diluents to render the oils more manageable and also as a means of temperature reduction, e.g., through the use of cold solvents.
In incremental dilution dewaxing, solvent is added in increments to the waxy oil and the mixture is indirectly chilled in double pipe heat exchangers, the internal surface of which is scraped using a scraper blade to prevent wax build up.
Alternatively, cold solvent can be directly injected into the waxy oil under conditions of high agitation to prevent shock chilling. A preferred embodiment is disclosed in U.S. Pat. No. 3,773,650, which describes a "dilution chilling" dewaxing method in which a waxy oil stock is introduced into a cooling zone divided into a plurality of stages. Dewaxing solvent is introduced into the cooling zone at a plurality of points along the cooling zone, coming into contact with the oil and forming a wax-oil-solvent mixture.
High levels of agitation are provided in at least a portion of the solvent-containing stages, thereby providing substantially instantaneous mixing of solvent and oil, i.e., within a second or less. As the oil passes through the cooling zone, it is cooled to a temperature sufficient to precipitate at least a portion of the wax therefrom, resulting in the formation of a wax slurry wherein the wax has a unique crystal structure with markedly superior filtering characteristics and wherein the wax slurry has a relatively high filtration rate and good dewaxed oil yields are obtained.
Alternatively, the procedure of U.S. Pat. No. 3,775,288 may be employed wherein lubricating oil fractions are dewaxed by contacting them with successive increments of cold solvent at a plurality of points along a vertical tower while maintaining a zone of intense agitation at each point of solvent injection so that substantially instantaneous mixing occurs at each point, continuing the chilling by means of said cold solvent injection until a temperature greater than the filtration temperature but less than about 35.degree. F. above the filtering temperature is reached and completing the cooling of oil to the filtration temperature in a scraped surface chiller.
In U.S. Pat. No. 4,146,461, an improved dilution chilling dewaxing process is described in which the temperature profile of the chilling tower is modified. In that process, waxy oils are solvent dewaxed by contacting them with successive increments of cold dewaxing solvent at a plurality of points along the height of a vertical tower divided into a plurality of stages while agitating the oil-solvent mixture in each stage to provide substantially instantaneous mixing of waxy oil and solvent thereby precipitating wax from the oil while avoiding shock chilling. The improvement resides in adjusting the cold solvent addition to each stage in a manner so as to modify the temperature profile along the tower to ensure that the temperature drop per stage in the initial stages in which wax precipitation occurs is greater than the temperature drop per stage in the final or later stage in which wax precipitation occurs.
U.S. Pat. No. 4,356,080 describes the solvent deoiling of slack waxes (and the separation of wax from oil in general) using scraped surface chillers into which cold solvent is injected into conduits transporting the waxy oil using injectors which produce turbulent mixing such that substantially uniform and instantaneous mixing of the injected solvent and the waxy oil is effected.